It is known to use compounds which exhibit a spin transition for various applications, in particular for information storage. Such compounds can in particular be coordination complexes comprising one or more metal centers having a 3d4, 3d6 or 3d7 configuration, one or more nitrogenous ligands and one or more anions, such as described, for example, in EP-0 543 465, EP-0 666 561, EP-0 745 986 and EP-0 842 988.
EP-0 543 465 describes a process for the preparation of spin transition compounds and the use for information storage. The process consists in bringing together, on the one hand, the ligand and, on the other hand, an iron salt in an acid solution, in allowing to react, in order to obtain a precipitate, and in recovering the precipitate in the powder form. For the use for data storage, the complex obtained is reduced beforehand to a powder in order to be deposited on a support by various methods. The compounds mentioned correspond to one of the following formulae:                FeL3(NO3)2 in which L is a ligand of the 1,2,4-triazole or 4-amino-1,2,4-triazole type, in combination with the NO3− anion;        Fe(ATP)2,5Cl2, in which the ATP ligand is 4-amino-1,2,4-triazole in combination with Fe(II) and with Cl−;        Fe(TP)2Cl2 in which the TP ligand is 1,2,4-triazole, in combination with Cl−;        [Fe(2-aminomethylpyridine)3]Cl2EtOH, EtOH being ethanol;        [Fe(1,10-phenanthroline)2](NCS)2;        [Fe(1-propyltetrazole) 6](BF4)2;        complexes of a metal M in combination with a mixture of several ligands (chosen from R-Trz, amines NL2 and triazolates Trz-, M being Fe(II), Fe(III) or Co(II), R-Trz being a triazole carrying an R substituent, R and L being an alkyl or H) and with an anion chosen from BF4−, ClO4−, CO32−, Br− and Cl−, the complex additionally comprising a defined amount of water.        
With the exception of [Fe(1,10-phenanthroline)2(NCS)2], all these complexes are pink in color in the low spin (LS) state and white in the high spin (HS) state. The transition is brought about by heating or cooling and takes place between −20° C. and 100° C. They exhibit a phenomenon of hysteresis which can range from a few degrees to a few tens of degrees.
EP-0 666 561 describes spin transition compounds which correspond to the formula Fe(II)(H-Trz)3(X)2 in which Trz is 1,2,4-triazole and (X)2 represents the anion (BF4−)2, (ClO4−)2, (Br−)2, (Cl−)2 or (CO32−). These compounds exhibit two crystalline phases, each having spin transitions associated with a change in color (white/pink) and for which the temperatures T1/2↓ and T1/2↑ are respectively less than and greater than ambient temperature. The preparation process is analogous to that described in EP-0 543 465 above.
EP-0 745 986 describes compounds corresponding to a formula analogous to that of the compounds of EP-0 543 465, in which M is a metal ion of d5, d6 or d7 configuration, the ligand is a dialkylaminotriazole and the anion comprises a sulfitoaryl, sulfitoalkyl, sulfitoaryl halide or sulfitoalkyl halide group. These specific compounds have a hysteresis amplitude of greater than 70° C. and a region of bistability centered exactly around ambient temperature. Said compounds are pink in the LS state and white in the HS state. The process for the preparation of the compounds, described very briefly, is analogous to that described in EP-0 543 465 above.
EP-0 842 988 describes spin transition chemical compounds and their use in display devices where a temperature threshold is exceeded. The compounds are formed by a network composed of molecules each formed by a metal-ligand complex and by an anion, and they comprise at least one water molecule bonded to the ligand via a hydrogen bond. The metal is chosen from those which have a d4, d5, d6 or d7 configuration. The ligand is 1,2,4-triazole carrying an R substituent comprising an OH group. The anions are nitrate and tosylate derivatives. The compounds corresponding to this definition have a temperature T1/2↑ of between 80 and 95° C. and a T1/2↓ of −170° C. They can be used in particular in devices intended to detect an accidentally high (of the order of 80° C.) storage temperature in storage buildings or transportation vehicles. The compounds are prepared by mixing a precursor of the metal center and a precursor of the ligand, at ambient temperature, and by removing the solvent by filtration after a precipitate has been obtained. The compound is obtained in the pulverulent form.
The compounds obtained according to the prior art above are of micrometric size and have to be ground in order to be usable as thermochromic pigments in polymer films having a micrometric thickness or as data carrier in Microsystems, the carriers having to remain transparent.